1. Field of the Invention
The present invention relates to a method of treating a surface of a substrate using modified urushiol derived from fresh Toxicodendron vernicifluum, and more particularly, to a method of treating a surface of a substrate using modified urushiol derived from fresh Toxicodendron vernicifluum which may exhibit high antibacterial activity and excellent surface properties (surface appearance and functionalities) such as far-infrared radiation, blocking of electromagnetic waves, enhanced corrosion resistance, high crosslinking speed obtained even when photoinitiator content is low, excellent surface gloss and high scratch resistance since reactivity of a hydroxyl group in urushiol extracted from fresh Toxicodendron vernicifluum is removed before urushiol is used as a UV coating agent for a substrate such as a steel sheet.
2. Discussion of Related Art
Toxicodendron vernicifluum, commonly known as Chinese lacquer tree grown in Central Asia and Himalaya, is usually produced in East Asian countries such as China, the Tropics, Japan and Korea. Oriental lacquer obtained from the trees is widely used as coating material for metals and wood due to its advantageous effects such as antibacterial activity, weather resistance, wear resistance, water resistance, corrosion resistance, etc., and is also used in Chinese medicine as a medical herb. It is known that the main ingredients of the raw lacquer are urushiol (60 to 70%), water (approximately 20%), other water- and oil-soluble proteins, laccase and gums.
Urushiol is known as a catechol having an unsaturated linear alkyl chain, and generally understood as the main cause of poison ivy rash. Such urushiol is known to easily dissolve in various polar and non-polar solvents, and thus a method of extracting urushiol using an organic solvent is well developed. As shown in FIG. 1, raw lacquer is composed of an urushiol monomer and an oligomer having a molecular weight of approximately 1,000 to 5,000 g/mol through an enzyme reaction of a laccase contained therein when the raw lacquer is in a natural state or left in the air.
Recently, many endeavors to solve the above-mentioned problems of urushiol, and develop coating and painting products having excellent antibacterial activity through polymerization and crosslinking of urushiol have been reported. However, a classic example is free radical polymerization in the presence of a laccase or peroxide initiators. While the former requires long polymerization and crosslinking time, the latter can complete the reaction within a few hours. Therefore, the latter has been studied and developed for various applications. According to Korean Patent Publication No. 2008-0045926, expensive urushiol was used in a wide range of weight ratio, for example, 100:1 to 100 based on UV paint.
However, it has been indicated that a catechol structure in urushiol has problems of decreased gloss of a coated surface, low polymerization speed, low cure rate, and high initiator content due to a side reaction (a radical capture reaction) shown in FIG. 2 on free radical polymerization (including both of thermal curing and photocuring).